Urban agriculture is playing an increasingly important role in global food security, a study has suggested.
Researchers, using satellite data, found that agricultural activities within 20km of urban areas occupy an area equivalent to the 28-nation EU.
The international team of scientists says the results should challenge the focus on rural areas of agricultural research and development work.
The findings appear in the journal Environmental Research Letters.
"This is the first study to document the global scale of food production in and around urban settings," explained co-author Pay Drechsel, a researcher for the International Water Management Institute (IWMI).
"There were people talking about urban agriculture but we never knew details. How did it compare with other farming systems? This assessment showed us that it was much larger than we expected."
The team acknowledged that the study could actually be conservative, as it focused on urban areas with populations of 50,000 or greater.
Dr Drechsel said that when urban farming was compared with other (ie rural) farming systems, the results were surprising. For example, the total area of rice farming in South Asia was smaller than what was being cultivated in urban areas around the globe.
Likewise, total maize production in sub-Saharan Africa was not as large as the area under cultivation in urban areas around the world.
UN data shows that more than 50% of the world's population now lives in urban areas, which could explain the changing landscape of global agriculture.
"We could say that the table is moving closer to the farm," observed Dr Drechsel.
"The most interesting factor when we look at India is that we could map the whole country as urban or peri-urban because there are so many towns and cities."
He added: "This has so many consequences in terms of what cities do to their environment because they are sucking out water but giving back polluted waste."
Using Ghana as an example, Dr Drechsel said that the majority of vegetable farmers irrigated their crops with polluted water. In Accra, it is estimated that up to 10% of household wastewater was indirectly recycled by urban farms.
"These farms are now recycling more wastewater than local treatment plants," he observed.
Lead author Anne Thebo from the University of California, Berkeley, said the study was "an important first step towards better understanding urban crop production at the global and regional scales".
She added: "In particular, by including farmlands in areas just outside of cities we can begin to see what these croplands really mean for urban water management and food production."
Dr Drechsel explained that there was a marked difference in attitudes between the developed world and developing nations when it came to urban agriculture.
"In the North, we consider agricultural activities in cities as something positive," he told BBC News.
"We think it is really useful and there are many models as to how we could better integrate agriculture into cities.
"Yet in the South, it is considered to be an oxymoron - farming and cities have nothing in common and they would like to get all of the farming out of the cities."
He explained that it was important to foster a greater level of integration between agricultural and urban development policies.
"This is not happening in large parts of the developing world because the urban sprawl is happening far too quickly. The legislative, administrative infrastructure is unable to keep pace."